At the time, a spokesman for the UK's Health Protection Agency (HPA), said: "If the material is outside of its protection packaging, anyone who has come into prolonged physical contact, such as keeping it in a pocket, should seek medical assistance."

A senior environment ministry official based in Basra told Reuters up to 10g (0.35 ounces) of Ir-192 "capsules" was taken.

Video:Nuclear Smuggling Arrests

The material is classed as a Category 2 radioactive source by the International Atomic Energy Agency and can be fatal if someone is exposed for a period of hours to days.

Security officials fear it could be used to make a so-called dirty bomb - an explosive device that renders part of a town or city unusable for generations by scattering nuclear material over a wide area.

A senior security official with knowledge of the theft, using the Arabic name for Islamic State, said: "We are afraid the radioactive element will fall into the hands of Daesh.

"They could simply attach it to explosives to make a dirty bomb."

The official, based in Baghdad, said there were no immediate suspects, but an initial investigation suggested the thieves had specific knowledge of the material and the facility.

The document obtained by Reuters said the material had gone missing from a US oilfield service company called Weatherford, but the company on Wednesday issued a statement denying it bore any "responsibility or liability".

It said another company called SGS was the operator of the depot and was "solely responsible".

The US State Department said it had "not seen any indication the material in question has been acquired by Daesh or any other terrorist groups in the region".